Technical Abstract:
Soil samples from a long-term experiment on tillage, residue, and nitrogen management under continuous corn (C4) and later cropped to soybean (C3) were analyzed for **13C and organic C to determine the changes in delta **13C and organic C as a result of the different management systems. The delta **13C values in the topsoil increased steadily with continuous corn and decreased following soybean. No-till plots with residue returned (NTS), particularly with added fertilizer N, had the greatest change and had the highest delta **13C values while the no-till plots with residue removed (NTO), especially without N fertilizer, had the lowest delta **13C values. Organic C contents in the topsoil remained about the same over time with continuous corn except for NTS which showed increases under corn then dropped following soybean. Both residue management and introduction of soybean greatly affected the distribution of **13C and organic C with soil depth in no-till plots but not in plowed plots. NTS plots exhibited the most dramatic changes in delta **13C and organic C with depth as a result of crop change from corn to soybean.